1. Field of the Invention
The present invention relates to system synchronization in video oculography based neuro-otologic testing and evaluation.
2. Background Information
A standard ophthalmic exam is a series of tests done to check a subject's vision and the health of the subject's eyes. Included in this testing are standard tests used to check visual acuity, pupil function, eye movement and peripheral vision. The basic format or operation of these tests are well known to the practitioners the field, some general (overlapping) categories used in the field to broadly describe such testing includes eye movement tests, visual acuity tests, Pupillometry tests, nystagmus tests, smooth pursuit tests, saccades tests, optokinetic tests, peripheral vision testing, subjective visual horizontal and subjective visual vertical tests.
Visual acuity tests may be performed in many different ways. It is a quick way to detect vision problems and is frequently used in schools or for other mass screening, e.g. military recruits. Driver license bureaus often use a small device that can test the eyes both together and individually.
Pupillometry tests represent conventional examination of pupillary function includes inspecting the pupils for equal size (1 mm or less of difference may be normal), regular shape, reactivity to light, and direct and consensual accommodation. A swinging flashlight test is one known pupillometry test which may also be desirable if neurologic damage is suspected. In a normal reaction to the swinging-flashlight test, both pupils constrict when one is exposed to light. As the light is being moved from one eye to another, both eyes begin to dilate, but constrict again when light has reached the other eye.
Eye movement testing can also be called extra-ocular muscle function testing is an examination of the function of the eye muscles. These tests observe the movement of the eyes in six specific directions.
Peripheral vision testing is also called visual field testing. It has been suggested that evaluation of the visual fields should never be omitted from the basic eye examination. Testing the visual fields consists of confrontation field testing in which each eye is tested separately to assess the extent of the peripheral field.
In many of the above testing, namely check visual acuity, pupil function, eye movement and peripheral vision, testing apparatus have been developed to automatically supply the appropriate visual stimulus to the subject to conduct the test. Further, these devices can be found in desk mounted arrangements which accommodate the subjects head, wall or desk mounted units (e.g. monitors) that provide that the subject is a fixed distance away typically in a chair, chair mounted projection type units mounted on the subjects chair that project onto the wall, and even head mounted units attached to the subject.
Within the meaning of this application any ophthalmic eye testing device that supplies a predetermined visual stimulus to the subject in a predetermined location (which may move) is an automated ophthalmic eye testing device. One such automated ophthalmic eye testing device is the laser based PURSUIT TRACKER® system of visual stimulus generator available from the applicant, Neuro-Kinetics, Inc. This type of device, and some of the testing that can be performed with such devices, is described in U.S. Pat. Nos. 7,651,224 and 8,333,472 which are incorporated herein by reference. Additionally the ophthalmic eye testing device may be incorporated with an eye tracker such as described in U.S. Pat. No. 8,585,609 which is incorporated herein by reference.
Eye trackers measure eye movement, i.e. rotations of the eye, in one of several ways, but principally they fall into three categories:
One category of eye tracker uses an attachment to the eye, such as a special contact lens with an embedded mirror or magnetic field sensor, and the movement of the attachment is measured with the assumption that it does not slip significantly as the eye rotates. Measurements with tight fitting contact lenses have provided extremely sensitive recordings of eye movement, and magnetic search coils are the method of choice for researchers studying the dynamics and underlying physiology of eye movement.
A second category of eye tracker uses electric potentials measured with electrodes placed around the eyes. The eyes are the origin of a steady electric potential field, which can also be detected in total darkness and if the eyes are closed. It can be modeled to be generated by a dipole with its positive pole at the cornea and its negative pole at the retina. The electric signal that can be derived using two pairs of contact electrodes placed on the skin around one eye is called Electro-oculogram (EOG). If the eyes move from the center position towards the periphery, the retina approaches one electrode while the cornea approaches the opposing one. This change in the orientation of the dipole and consequently the electric potential field results in a change in the measured EOG signal. Inversely, by analyzing these changes in eye movement can be tracked. Due to what is known as the discretization given by the common electrode setup two separate movement components—a horizontal and a vertical—can be identified. The potential difference is not constant and its variations make it challenging to use EOG for measuring slow eye movement and detecting gaze direction. EOG is, however, a very robust technique for measuring saccadic eye movement associated with gaze shifts and detecting blinks. It is a very light-weight approach that, in contrast to current video-based eye trackers, only requires very low computational power, works under different lighting conditions and can be implemented as an embedded, self-contained wearable system. It is thus the method of choice for measuring eye movement in mobile daily-life situations and REM (Rapid Eye Movement) phases during sleep.
The third broad category of eye tracker, which is the category relevant to the present invention, uses some non-contact, optical method for measuring eye motion. Generally these are video based eye trackers. Light, typically infrared, is reflected from the eye and sensed by a video camera or some other specially designed optical sensor. The information is then analyzed to extract eye rotation from changes in reflections. One class of video based eye trackers typically uses the corneal reflection (the first Purkinje image) and the center of the pupil as features to track over time. A more sensitive type of eye tracker, the dual-Purkinje eye tracker, uses reflections from the front of the cornea (first Purkinje image) and the back of the lens (fourth Purkinje image) as features to track. A still more sensitive method of tracking in this class is to image features from inside the eye, such as the retinal blood vessels, and follow these features as the eye rotates. Other video systems digitize the eye and locate the pupil in the image and utilize object recognition analysis or processing to locate and track the pupil in the digitized image. Optical methods, particularly those based on video recording, are widely used for gaze tracking and are favored for being non-invasive and inexpensive.
With this general background, the problems addressed by the present claimed invention can be described. Many of the ophthalmic test parameters represent physiologic parameters only obtainable with a high speed devices, and thus only became possible in VOG systems with cost effective high speed cameras. These parameters, and other “high speed parameters” such as second order or higher corrective saccadic eye movements described in U.S. Patent Publication 2012-0081666 which is incorporated herein by reference, and micro-saccades (defined or described as involuntary saccades produced during attempted fixation. see the CALTECH article “Microsaccades: a Neurophysiological; Analysis” by Susana Martinez-Conde of the Barrow Neurological Institute, Trends in Neurscience Volume 32 No. 9, Pgs 463-475, 2009.), Drifts (described as “slow curvy motions between microsaccades), Tremors (described as very fast (about 90 Hz) small oscillations superimposed upon drifts) etc, raise a new issue with such video based systems, namely the synchronization of the actual timing of the visual stimulus with measurements associated with that timing.
Basically measurement of these high speed parameters in a video based eye tracking system can become inaccurate due to the processing delays within the conventional computer without a synchronization system. There is a need in the art, system synchronization in video oculography based neuro-otologic testing and evaluation.